High frequency electron discharge devices and thermionic cathodes having improved (cvd) refractory insulation coated heater wires



March 5, 1968 c, s, PEARSALL ET AL 3,372,297

HIGH FREQUENCY ELECTRON DISCHARGE DEVICES AND THERMIONIC CATRoDEs HAVING IMPROVED (CVD) REFRACTORY INSULATION COATED HEATER wIREs Filed Sept. 28. 1964 vFIGI 1 2 F||G.2 6 l5 '6 A 7 m 4 I .FIG 4' CHEMICAL VAPC l DE 08 2 5 A f l E E LLI 19E. H2 C0 ,20 I

I I oo o I oo g emRE TEMPER- 4| DRYER DRYER ,42

)8 H65 CHEMICALVAPOR H66 DEPOSITION FLOW FLOW ELECTROPHORE- Tl CALlX COATING RESISTANCE (ME' GOHMg) I380 I420 I460 I500 FIG? if 9 HEATER WIRE TEMPERATURE O r I C. i I INVENTORS I CORTLAND s. PEARSALL BY RAY I WIMBER 4 VARIABLE fin. A.C.POWER 43 I6 5/ ATTORNEY United States Patent HlGH FREQUENCY ELECTRON DISCHARGE DE- VICES AND THERMIONIC CATHODES HAVING KMPROVED (CVD) REFRACTGRY INSULATION COATED HEATER WIRES Cortland S. Pearsall, Los Altos, and Ray T. Wimber, San Diego, Calif, assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed Sept. 28, 1964, Ser. No. 399,461 17 Claims. (Cl. 313-337) ABSTRACT OF THE DISCLOSURE The heat transfer adherence, coating resistance, density and electrolytic degradation characteristics of a coated heater for use in a thermionic cathode are improved by utilizing a chemical vapor deposited (CVD) refractory insulation coating on a refractory metal heater wire. The terminology (CVD) is restricted to include only coatings which are formed by the deposition of a non-volatile compound on a selected substrate from a gasv (vapor) or mixture of gases, said mixture containing all the chemical elements required to synthesize the desired compound in the form of one or more gaseous compounds differing from the following compound. The deposition may take place either by thermal decomposition of a gas or by chemical reaction of the gas mixture constituents at the substrate which has been raised to a suitable temperature.

This invenion relates in general to electron discharge devices and more particularly to such devices having improved cathode structures therein.

An integral part of electron discharge devices such as backward wave oscillators (BWO), traveling wave tubes (TWT), klystrons, magnetrons, triodes, pentodes, etc., is the electron emitting or cathode portion thereof. Refractory metal heaters, such as for example, tungsten, are conventionally used to provide a source of heat for thermionic cathodes. The heat transfer between the refractory metal heater and the thermionic cathode portion of an electron gun is a critical problem and large research ex penditures are constantly being made in order to optimize such heat transfer. Generally speaking, the prior art has taught that an insulated refractory metal heater is superior to a non-insulated refractory metal heater with regard o the prevention of shorting between turns of the heater or to the surrounding cathode sleeve and with respect to the prevention of cathode failure due to environment causes such as vibration, etc.

However, serious problems in reduction of thermal emissivity and electrical resistance are inherent in coated heaters. Further problems such as chipping, cracking, etc. are present in heaters having insulation coatings thereon, with the consequent danger of cathode failure being present. Conventional techniques employed to coat refractory metal heaters are the utilization of refractory insulations such as alumina (aluminum oxide) on the refractory metal heater by electrophoretically depositing and sintering (EDS) processes. Resistance to chipping and wear, as well as the efficiency of thermal emissivity are not optimized in such electrophoretically deposited and sintered (EDS) alumina powdered coatings.

In a preferred embodiment of the present invention, the novel combination of a chemical vapor deposited (CVD) refractory insulation coating, such as alumina, on a refractory metal heater provides improved electrical resistance of the coating, greater thermal emissivity, less volatilization associated With the higher purity of the CVD coatings, and considerably greater adherence and resistance to chipping than prior art (EDS) alumina powders; Also, the resistance of the CVD refractory insulation coatice ing to electrolytic degradation under the influence of a DC. potential difference is much greater than that of EDS coatings.

In the chemical vapor deposition of a non-volatile compound (or element) on a substrate, the elements constituting the deposit are transported to the substrate in the form of one or more gaseous (vaporized) compounds chemically different from the deposit itself. Deposition may take place either by thermal decomposition or chemical interaction of the gas phase constituents at the substrate. Therefore, chemical vapor deposition (CVD) as used in the following specification and claims is herein defined to mean: the deposition of a non-volatile compound on a selected substrate from a gas (vapor) or mixture of gases, said mixture containing all the chemical elements required to synthesize the desired compound, in the form of one or more gaseous compounds differing from the final compound. Deposition may take place either by thermal decomposition of a gas or by chemical reaction of the gas mixture constituents at the substrate which has been raised to a suitable temperature. Also, for purposes of definition, a refractory metal may be defined as a metal element or metal alloy having a melting point greater than 15 00 C. and a refractory insulation will have a meltitng point greater than 1500 C.

It is therefore an object of the present invention to provide electron discharge devices utilizing thermionic cathodes with improved insulated refractory metal heaters in the cathode portion thereof.

A feature of the present invention is the provision of an electron discharge device utilizing a refractory metal eater with a chemical vapor deposited refractory insulation coating thereon.

Another feature of the present invention is the provision of a thermionic cathode assembly having a refractory metal heater which includes a chemical vapor deposited refractory insulation coating on said heater.

Another feature of the present invention is the provision of a refractory metal heater having a chemical vapor deposited refractory insulation coating wherein said insulation coating is selected from the group consisting of aluminum oxide, beryllium oxide and yttrium oxide.

These and other features and advantages of the present invention will become more apparent after a perusal of the following specification taken in conjunction with the accompanying drawing wherein:

FIG. 1 is an elevational view, partly in section, depicting an illustrative electron discharge device of the BWO type which incorporates the novel features of the present invention;

FIG. 2 is an elevational view, partly cut-away and partially sectioned depicting the electron gun portion of the electron discharge device depicted in FIG. 1 taken along the lines 2-2 of FIG. 1;

FIG. 3 is a pictorial view fragmented and sectioned, taken along lines 33 of FIG. 2;

FIG. 4 is an illustrative graphical portrayal of heater power vs. heater wire temperature showing the characteristics of CVD refractory insulation coatings in comparison with EDS prior art coatings utilized on refractory metal heaters.

FIG. 5 is an illustrative graphical portrayal of coating resistance vs. heater wire temperature for the CVD refractory insulation coating vs. EDS insulated cooatings of the prior art.

FIG. 6 is a schematic representatioon, partly in section and partly in elevation, of a suitable apparatus utilized to provide the CVD refractory insulation coating on heaters as taught by the present invention.

FIG. 7 depicts a cathode having (CVD) insulation on the support sleeve and rear portion of the emission surface.

Referring now to FIG. 1 there is depicted an electron discharge device 1, one of the BWO type, and the focusing structure 2 utilized to control the electron beam along the axial extent of the BWO. For a more detailed explanation of the operation and construction of a BWO such as depicted in FIG. 1, see U.S. Patent No. 2,991,391 by W. L. Beaver. The electron gun portion delineated by lines 2-2 of the BWO depicted in FIG. 1 is shown in detail in FIG. 2, and reference thereto is now made.

Electron gun portion 3 is supprted within tubular body extension member 4 through the utilization of supporting techniques generally similar to those depicted in the aforementioned U.S. Patent No. 2,991,391. The electron gun 3 includes a plurality of preferably sapphire insulating and mounting rods 5 upon which three preferably molybdenum annular mounting members or washers 6, 7 and S are mounted as shown. Annular washer 7 is dish-shaped at its central portion and serves as an outer focusing electrode for the electron beam emanating from a thermionic cathode assembly having an annular cathode button 9. A preferably molybdenum cathode support sleeve 10 serves as a rigid mountin and support means for the annular cathode button 9, as shown. A central focusing electrode 11 is fixedly secured to the central portion of the annular cathode button 9 as shown. The support sleeve 10 is mounted within an annular sleeve member 12 as shown and a pair of heat shields 13 and 14 fixedly secured to the washer 8 and to each other complete the visually significant portions of the particular electron gun assembly depicted in FIG. 2. An annular grid support member 15 is secured to washer 6 such as by brazing and a hex grid 15 is mounted thereon as shown. Disposed within the cathode support sleeve 10 immediately below the cathode button 9 is a coated refractory metal heater 16 having a CVD refractory insulation coating thereon as taught by the present invention and as best seen in FIG. 3.

Generally, a snug fit between the exterior periphery of the coated heater 16 within the annular mounting sleeve 10 is desired in order to optimize heat transfer to the button 9 and to minimize vibration problems which would otherwise be a hazard in mobile environments. The heater depicted in FIG. 3 is pictorially representative of both the prior art, EDS alumina coating as well as the CVD alumina coating of the present invention.

In FIG. 6 apparatus 18 for providing CVD coatings on heaters utilizable in electron gun portions of electron discharge devices is depicted.

The apparatus 13 includes a conventional gas cylinder type source of hydrogen gas 19 under pressure and a similar source of carbon dioxide gas 20 under pressure, which can be coupled through conventional control valves 21, 22 and through conventional flow meters 23 and 24 after the gases have preferably been passed through conventional type dryers, such as plastic cylinders 41, 42 packed with a suitable drying agent (desiccant) such as anhydrous calcium sulphate which eliminates Water vapor which may be present in the gases. Suitiable tubulations 25 such as, for example, of Pyrex glass or the like may be utilized to interconnect the aforementioned elements. The hydrogen and carbon dioxide gas emanating from the flow meters 23, 24 is combined in tubulation 26 whereupon the combined gas enters inlet part 27 of vaporizer 28 which is preferably of Pyrex. Aluminum chloride granules 29 line the bottom portion of the vaporizer 28 as shown. Suitable heater means represented by heating mantel 30, the top of which is preferably packed with fibrous insulation 30; are utilized to provide the required heat in the vaporizer portion 28. Outlet tubulation 31 extends to a reaction chamber 32 which is preferably of Pyrex wherein the refractory metal heater 33 is supported therein as shown. A suitable condenser 34 which may be operated at room temperature is coupled as shown to the reaction chamber. A variable AC. power supply 35 is coupled to the support elements 36, 37 upon which the heater 33 may be spot welded or mechanically clamped during the coating process.

The entire system of vaporizer, interconnecting tubulations, reaction chamber and condenser is operated at atmospheric pressure and any suitable sized exhaust tubulation 40 attached to condenser 34 can be used to operate the system at atmospheric pressure.

In operation, H and CO carrier gases emanating from sources 19 and 20 are coupled through dryers 41, 42 and tubulations 25 and rate and ratio controlled by means of control valves 21, 22 and then coupled to combiner tubulation 26 and thence into the vaporizer 28 where vaporization of the aluminum chloride, AlCl occurs and the resultant gaseous mixture flows through outlet tubulation 31 and thence into reaction chamber 32. The gaseous mixture flows down reaction chamber 32 and deposits an aluminum oxide coating on the refractory heater member 33 supported therein as shown. The CVD of alumina on a refractory metal heater may be described by the following overall reaction taking place on the heater surface of the heater element:

The residual gases then are exhausted through the tubulation connecting the condenser and reaction chamber 32 whereupon the excess aluminum chloride is condensed on the walls of the condenser.

In a typical example of the process for CVD of the refractory insulation coating the following operating conditions were established in order to deposit a (1 mil) thick refractory insulation coating on a 6 mil diameter refractory tungsten heater wire. The control valves 21 and 22 were set to provide 50 cubic centimeters/ minute of H and 16 cubic centimeters/minute of CO within the vaporizer 28 at a vaporizer temperature of 180 C. Under these conditions 255 milligrams of aluminum chloride/minute were vaporized and expelled through outlet tubulation 31 into reaction chamber 32. A deposition time of 25 minutes was required under the above conditions, operating the system at atmospheric pressure, in order to obtain a 1 mil thick coating on the heater wire 33 heated to an elevated temperature.

A resistance heating technique utilized a variable AC. power supply 35 coupled to support leads 36, 37 to provide the necessary temperature for the heater element 33 disposed within reaction chamber 32. In the aforementioned operating conditions an optical pyrometer, not shown, was used to set the heater temperature to approximately C. A 20 mesh screen was used to provide plus 20 granule size of A101 in the vaporizer 28. Although resistance heating techniques are depicted in the apparatus shown in FIG. 6 for providing the required heater temperature in reaction chamber 32 during deposition it is to be understood that induction, radiation or other heating techniques may be advantageously employed without departing from the scope of the present invention. For example, the heater specimen, to be coated, would be heated indirectly using a secondary heat source either inductively or self-resistance heated. The heater specimen, to be coated, would be surrounded by the secondary heat source such as a conductive sleeve which upon heating would radiate heat to the heater specimen to provide a uniform temperature for the specimen during the deposition process.

The comparative results, shown in FIG. 4, with respect to 1 mil thick CVD refractory insulation coating deposited as described above and 1 mil thick EDS coatings on equal 6 mil diameter coiled tungsten heaters, were observed. With respect to FIG. 4 a comparison of CVD coating with EDS coatings, with regard to heater power and heater temperatures for coated heaters radiating freely to the cold walls of a vacuum bell jar are shown. It is obvious upon examination of FIG. 4 that the CVD coating of aluminum oxide for a given heater wire temperature in degrees centigrade, which is indicative of the thermal emissivity efficiency for the power being utilized, is much greater with respect to heat radiation than the equivalent EDS coatings. For example, with a heater input power of 8 watts, the CVD coatings ran 250300 C. cooler than the EDS coating during comparative tests.

FIG. 5 depicts a graphical comparison between CVD coated heaters and EDS coated heaters. The characteristics depicted in FIG. 5 are coating resistance in megohms vs. heater wire temperature which provides an indication of the amount of leakage resistance which is inherent in both embodiments and thus the relative effectiveness of the particular coatings as insulation means. At a heater temperature of approximately 1420 C. it is observed that approximately a 40 megohm differential exists between the CVD and the EDS coatings. Thus, it is readily apparent that the CVD type of refractory insulation coating is superior to the EDS type both from thermal emissivity and insulation resistance viewpoints.

Further observations and tests have shown that the CVD refractory insulation coating is appreciably less volatile than the EDS coatings and thus the danger of cathode poisoning is greatly reduced through utilization of the CVD refractory insulation coating of the present invention. Furthermore, the resistance to chipping and strength factors of the CVD refractory insulation coating in comparison with the EDS type of refractory insulation coating show that the CVD refractory insulation coating is much stronger and much more resistant to chipping than the EDS refractory insulation coating. This, of course, results in less danger of shorting when a heater is utilized in a vibratory embodiment such as found in many mobile applications wherein electron discharge devices are employed. Present tests have shown conclusively that a CVD refractory insulation coating prepared according to the aforementioned parameters is greater than 2% times stronger than a comparable EDS coating. With regard to the crushing, strength, tests have further shown that the CVD coated heater may be fixedly secured within the cathode support sleeve without danger of chipping, etc., which is an ever present possibility with conventional EDS coatings.

Preferably a granular reagent grade aluminum chloride is screened, using a 20 mesh-stainless steel screen before the plus 20 mesh fraction is charged to the vaporizer. Experiments with regard to variations of the individual parameters of the deposition process have shown that increasing deposition time will result in an increase of coating thickness. Furthermore, increases in vaporizer temperature also are found to result in increased coating thickness per given unit of time. The overall flow rates and ratios employed in the prior example have been varied with varying degrees of success. However, it has been shown that H to CO ratios of compared to the 3 used with the aforementioned example and lower AlCl to CO mole ratios (1 or less compared to the 2 used in the aforementioned example) will provide enhanced coatings with regard to uniformity of deposition.

The present invention can furthermore be enhanced with regard to dilution of the reactant atmosphere. The utilization of carbon monoxide and nitrogen carrier gases in addition to the CO and H carrier gases has shown that good uniform coatings may be obtained in this manner also.

Deposits of aluminum oxide were made on the surfaces of heaters self-resistant heated to optical pyrometer temperatures of 8501325 C. for periods of time varying between 9 and 45 minutes. In general, the coatings deposited on tungsten coils at temperatures in excess of 1000 C. are extremely non-porous (approach theoretical density) wherein theoretical density in the case of aluminum oxide can be based on the naturally occurring form of aluminum oxide which is commonly known as sapphire, nearly transparent, and very adherent. The electrical tests depicted in FIG. 5 are for refractory metal heaters coated by CVD at 1125 C. Testing at different temperatures in the range between 1100 and 1500" C. involved measuring the leakage of current from the tungsten wire substrate through the coating to a closely fitting nickel sleeve or finetungsten wire probe as well as the power dissipated by the heater with and without the surrounding sleeve. At awire temperature of 1500 (3., determined from the ratio of hot and cold resistance values, the CVD heater inside the nickel sleeve showed a coating resistance of 5.2 megohms whereas the EDS coating under the same circumstances showed a resistance of only 2.3 megohms. At the same temperature without the nickel sleeve the CVD heater dissipated 17 watts whereas the EDS heater dissipated only 7.9 watts. These tests and others indicated that the CVD refractory insulation coatings are reproducibly superior to the EDS refractory insulation coatings in the following ways:

(1) Greater electrical resistance,

(2) Greater thermal emissivity,

(3) More adherent and chip resistant,

(4) Cleaner, the CVD coating lost no material to the heater sleeve while the EDS coating typically transferred appreciable quantities of material to its surrounding sleeve,

(5) Ability to survive electrolytic degradation under the influence of D.C. potentials is enhanced,

(6) Greater density (approach theoretical density) and are therefore extremely non-porous,

(7) Nearly transparent in visible spectrum (useful coatings range from semi-translucent to transparent-transparent has been found to give optimum performance).

The following table is representative of the preferred operating ranges for the variable parameters involved in the depositions system depicted in FIG. 6.

H CO vol. ratios 3/1 to 10/1. Flow velocities of gaseous mate- 16 to cm./ min.

rial in reaction chamber 1 to 2. AlC1 /CO mole ratios C. to C. Vaporizer temperature ranges 1000 to 1350 C. Optical pyrometer (heater) tem- Dependent on desired perature ranges coating thickness for Deposition time any fixed set of the above.

It is to be noted that test runs at less than 1000 C. optical pyrometer temperatures were run but a marked reduction in coating strength of the alumina was observed. A preferred insulation thickness for a 6 mil coiled tungsten heater is around1-3 mils.

In order to prevent the formation of dendritic (treelike) growth, it has been found desirable to bring the vaporization temperature to the desired operating level and to equilibrium before the heater is brought to temperature and then bring the heater to be coated up to the operating temperature. The condenser 33 is preferably employed in order to prevent back diffusion of either atmospheric H O or 0 through the reactor exhaust and in order to provide a cool surface area in which the aluminum chloride can condense without plugging up a small sized conventional type outlet.

The present invention is particularly useful in providing chemical vapor deposited refractory insulation coatings of aluminum oxide on coiled or non-linear type heaters of refractory metals which metals include tungsten, tantalum, molybdenum, rhenium and the platinum metals, namely: platinum, palladium, rhodium, osmium, ruthenium and iridium and the various alloys of these metals which result in useful refractory metals for heaters in thermionic. cathodes which, partically speaking, could run into astronomical variations. A few of the more useful alloys will be specified for purposes of illustration only: tungsten-rhenium; platinum-iridium; platinum ruthenium; rhodium-iridium; rhodium-ruthenium, etc.

Aluminum oxide (A1 0 beryllium oxide (BeO) and yttrium oxide Y O are advantageously employed with any of the aforementioned refractory metals as a refractory insulation coating.'Aluminum oxide is preferred for various reasons. However, beryllium oxide and yttrium oxide may, in certain instances, be preferable and are within the scope of the present invention. Deposition parameters similar to the aforementioned may be utilized for depositing beryllia by the substitution of suitable beryllium compound granules such as BeCl for the AlCl granules. Yttrium oxide (Y O is deposited by using the above deposition techniques with the substitution of yttrium chloride (YCl granules for the AlCl granules and by raising the vaporizer temperature to above 700 C. and, if desired, by adding an inert gas such as, for example, argon as an additional carrier gas. The advantageous properties of transparency, thermal emissivity, density, non-porousness, etc., set forth previously in detail with regard to Al O will also be present in the CVD refractory insulation coatings of BeO and Y O Furthermore, the molybdenum sleeve surrounding the heater 16 and cathode button 9 may advantageously be coated with a refractory insulation coating 43 according to the teachings of the present invention as shown in FIG. 7 with the resultant advantageous benefits of improved insulation characteristics and further minimization of the possibility of shorting of heater surfaces to the molybdenum sleeve interior surfaces. The method of applying a chemical vapor deposited refractory coating to the interior portion of the cathode sleeve would be to extend the tubulation 31 by an auxiliary attachment thereto so that the reactive gases emanating therefrom would be directed into the interior portion of the sleeve 10 in order to provide a uniform coating by uniformly directing the gas over the interior surfaces of the sleeve. The sleeve 10 could advantageously be mounted on support stems 36, 37 in much the same fashion as the heater 33 either by spot welding mechanical clamping or by any other conventional technique. In Vapor Plating, the Formation of Coatings by Vapor Deposition Techniques by C. F. Powell, I. E. Campbell and B. W. Gonser, John Wiley & Sons, Inc., New York and Oxides Chapman and Hall, Ltd. London (1955), chapter 7, pp. 136-43, an analysis of techniques for deposition of aluminum oxide is outlined.

Examples of typical cathode assemblies utilizing refractory metal heaters which can advantageously benefit from the teachings of the present invention with regard to applying a CVD refractory insulation coating on the heater if a coated heater is desired are to be found in Materials and Techniques for Electron Tubes by Walter H. Kohl, Reinhold Publishing Corporation, New York,

NY. and Chapman and Hall, Ltd. London (1960).

chapter and in particular, cathode assemblies of the types depicted in FIGS. 15.7, 15.8, 15.11 and 15.12. Other examples of cathode types which may advantageously benefit from the use of a CVD coated heater are to be found in US. Patents 2,543,728 of H. J. Lemmens et al.; 2,624,024 of M. J. Jansen et al.; 2,663,069 of G. A. Espersen; 2,673,277 of H. J. Lemmens et al.; 2,698,913 of G. A. Espersen; 2,700,000 of R. Levi et al.; 2,869,017 of R. Levi; 2,878,409 of R. Levi; 2,107,945 of A. W. Hull et'aL; 2,131,589 of W. Espe and 2,723,363 of V. J. de Sanits et al. The aforementioned list of exemplary cathode structures and electron discharge devices employing thermionic cathodes is obviously not meant to be all inclusive but merely to illustrate various types of cathodes utilizing refractory metal heaters which could advantageously benefit from the use of a refractory insulation coating of the CVD type as taught by the present invention if a coated type of heater is desired.

Quite obviously the CVD insulation coating techniques of the present invention are most advantageously applicable to coiled, wound, or otherwise deformed, with respect to a straight line, types of refractory metal heaters incorporated in thermionic cathodes.

Since many changes can be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electron discharge device including a cathode assembly having a refractory metal heater, said refractory metal heater having a chemical vapor deposited refractory insulation coating thereon, said chemical vapor deposited refractory insulation coating selected from the group consisting of aluminum oxide, beryllium oxide and yttrium oxide.

2. The device as defined in claim 1 wherein said refractory metal heater is principally composed of tungsten and wherein said chemical vapor deposited refractory insulation coating is aluminum oxide.

3. The device as defined in claim 1 wherein said refractory metal heater has at least a portion thereof made from a metal or an alloy of a metal selected from the group consisting of tungsten, tantalum, molybdenum, rhenium, platinum, palladium, rhodium, osmium, ruthenium and iridium.

4. The device as defined in claim 1 wherein said heater is surronded by a sleeve member and said sleeve member has a chemical vapor deposited refractory insulation coat ing deposited on the internal portions thereof.

5. The device as defined in claim 1 wherein said cathode assembly includes an electron emissive cathode button supported by a sleeve member, said heater being disposed within said sleeve member in close proximity to said button, the internal exposed portions of said sleeve and said button having a chemical vapor deposited refractory insulation coating thereon.

6. A cathode assembly including a refractory metal heater and an electron emissive cathode means disposed in heat exchanging relation thereto, said refractory metal heater having a chemical vapor deposited refractory insulation coating thereon, said chemical vapor deposited refractory insulation coating selected from the group consisting of aluminum oxide, beryllium oxide and yttrium oxide.

7. The cathode assembly defined in claim 6 wherein said refractory metal heater is principally composed of tungsten, and wherein said chemical vapor deposited refractory insulation coating is aluminum oxide.

8. The cathode assembly defined in claim 6 wherein said refractory metal heater has at least a portion thereof made from a metal or an alloy of a metal selected from the group consisting of tungsten, tantalum, molybdenum, rhenium, platinum, palladium, rhodium, osmium, ruthenium and iridium.

9. The cathode assembly defined in claim 6 wherein said heater is surrounded by a sleeve member and said sleeve member has a chemical vapor deposited refractory insulation coating deposited on the internal portions thereof.

10. The cathode assembly as defined in claim 6 wherein said electron emissive cathode button is supported by a sleeve member, said heater being disposed with said sleeve member in close proximity to said button, the internal exposed portions of said sleeve and said button having a chemical vapor deposited refractory insulation coating thereon.

11. A refractory metal heater for use in a thermionic cathode including a refractory metal heater wire, said wire having a chemical vapor deposited refractory insulation coating deposited thereon selected from the group consisting of aluminum oxide, beryllium oxide and yttrium oxide.

12. The heater defined in claim 11 wherein said refractory metal heater has at least a portion thereof made from a metal or an alloy of a metal selected from the group consisting of tungsten, tantalum, molybdenum, rhenium, platinum, palladium, rhodium, osmium, ruthenium and iridium.

13. A refractory metal heater for use in thermionic cathodes including a coiled refractory metal heater wire having a chemical vapor deposited refractory insulation coating thereon.

14. A thermionic cathode for use in electron discharge devices including a refractory metal heater having a refractory insulation coating thereon, said refractory insulation coating being characterized by having a melting point greater than 1500 C. and falling within the following range of optical characteristics with respect to the visible spectrum:

semi-translucent to transparent.

15. The cathode defined in claim 14 wherein said refractory insulation material is further characterized by being highly dense and non-porous and by having a leakage resistance greater than 15 megohms when heated to 1400 C.

16. A refractory metal heater element having a refractory insulation coating thereon, said refractory insulation coating being characterized by having a melting point greater than 1500" C. and by falling within the following 10 range of optical characteristics with respect to the visible spectrum:

semi-translucent to transparent.

17. The heater element defined in claim 16 wherein said refractory insulation material is further characterized by being highly dense and non-porous and by having a leakage resistance greater than 15 megohms when heated to 1400 C.

References Cited UNITED STATES PATENTS 2,665,998 1/ 1954 Campbell et al. 117-106 X 2,700,626 1/ 1955 Mendenhall 117-106 2,714,563 8/1955 Poorman et al 117-105 2,817,784 12/1957 Katz 313-340 2,991,391 7/1961 Beaver 315-35 3,029,360 4/ 1962 Etter 313-340 3,178,308 4/1965 Oxley et al. 313-341 X 3,251,337 5/1966 Latta et al. 117-106 JOHN W. HUCKERT, Primary Examiner. A. J. JONES, Assistant Examiner. 

